Recently, people are more concerned about environmental problems and depletion of natural resources and, as such, interest in solar cells as an alternative energy source which does not cause environmental pollution is growing. Solar cells are classified into silicon solar cells, thin film-type compound solar cells, layered-type solar cells and the like. Among these solar cells, silicon semiconductor solar cells have been studied the most widely.
Among these solar cells, recently, thin film type compound solar cells are actively studied and developed.
Among thin film type compound semiconductors, Cu(In1-xGax)(SeyS1-y) (CI(G)S), which is a Group I-III-VI compound included in ternary compounds, has a direct transition type energy band gap of 1 eV or more and high light absorption coefficient. In addition, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is an electro-optically stable. Thus, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is an ideal material as a light absorption layer of solar cells.
CI(G)S based solar cells are made by forming a light absorption layer having a thickness of several microns. As methods of manufacturing the light absorption layer, a vacuum deposition method which does not require a precursor, a sputtering method which forms a CI(G)S thin film through heat treatment after forming a thin film with a precursor and an electrodeposition method were introduced. Recently, an ink coating method was introduced. According to the ink coating method, under non-vacuum, a precursor material is coated and then the coated material is heat-treated. Among these light absorption layer manufacturing methods, studies into the ink coating method are actively conducted since, by using the ink coating method, process costs may be reduced and a large area may be uniformly manufactured. Precursors used in the ink coating method may be various compounds or metals such as metal chalcogenide compounds, bimetallic metal particles, metal salts, metal oxides or the like.
In particular, when a metal chalcogenide compound is used as a precursor, a mixed Cu—Se and In—Se compound or synthesized CuInSe2 single-phase particles are used. When the mixed particles are used, a coating layer having a partially non-uniform composition may be formed. When the CuInSe2 single-phase particles are used, long reaction time for particle growth is required.
Therefore, there is a high need to develop a technology for a precursor which may form a highly efficient light absorption layer having an entirely more uniform composition, being stable against oxidation and having increased film density.